The present invention relates generally to a method of processing glass substrates, and more particularly to a method of annealing amorphous silicon films deposited on glass substrates.
Thin film transistors are made on large glass substrates or plates for use in monitors, flat panel displays, solar cells, and the like. The transistors are made by sequential deposition of various thin films, including amorphous silicon (both doped and intrinsic), silicon oxide, and silicon nitride. These thin films can be deposited by chemical vapor deposition (CVD).
A CVD process may require that the substrates withstand temperatures on the order of 280-400.degree. C. CVD processing has found widespread use in the manufacture of integrated circuits in silicon wafers. Silicon is a conductive material, and it can be heated and cooled quite rapidly without breaking or warping the wafer. However, glass is a dielectric material that is very brittle and is subject to warping or cracking when cooled or heated too rapidly. Thus, great care must be taken to adjust the rate of heating or cooling of large area glass substrates to avoid thermal stress and resulting damage.
Typically, to carry out the CVD process, the substrate is preheated in a heating chamber to about the deposition temperature. Once the substrate reaches the desired temperature, it is transferred into a processing chamber for deposition of the film. Then the substrate is transferred to a cooling chamber to cool it, e.g., to room temperature. To reduce the danger of contamination, the heating, deposition and cooling chambers may be included in a single tool so that the substrate may be transported between chambers without being removed from a vacuum environment. Such a tool, e.g., the system described in U.S. Pat. No. 4,951,601, which is incorporated herein by reference, may include a central robotic chamber connected to various processing chambers. The processing chambers may hold only a single substrate to improve process uniformity and controllability. However, due to the lengthy period of time required to heat up and cool down the temperature of a glass substrate (e.g., about five minutes each to heat a large area glass substrate to about 400.degree. C. and to cool it back to room temperature) to avoid damage or warpage to the substrate, the heating and cooling chambers may hold several glass substrates at the same time to improve the throughput of the system.
One step in the fabrication of a thin film transistor is the formation of a polycrystaline silicon layer. One process of forming a polycrystaline silicon layer begins with the deposition of an amorphous silicon precursor layer on the substrate by CVD. Following the deposition step, the substrate is removed from the CVD tool and transported to a furnace for annealing, e.g., at temperatures of about 350-400.degree. C. The annealing temperature is lower than the deposition temperature. Thereafter, the amorphous silicon may be converted to polycrystaline silicon by laser annealing.
One problem encountered in the fabrication of polycrystaline silicon is blistering of the substrate surface during the laser annealing step. This blistering renders the substrate unusable and reduces process yield. One cause of blistering is the outgassing of hydrogen which was incorporated in the amorphous silicon precursor layer during the deposition step. To reduce the amount of hydrogen incorporated in the precursor layer, the amorphous silicon deposition could be performed at a "high" temperature, e.g., at 450-470.degree. C. Unfortunately, such high temperatures are destructive to the process chamber.